1. Field of the Invention
Example embodiments of the present invention relates in general to the field of a driving circuit and method which are used in an Organic Light Emitting Diode (OLED), and more specifically to a driving circuit of an organic light emitting diode and a driving method thereof which can use a thin film transistor (TFT) as an active device.
2. Description of the Related Art
An Organic Light Emitting Diode (hereafter referred to an OLED) display device is a self-light emitting display device which displays images by electrically exciting a luminescent organic component to emit light, and has an advantage of a low driving voltage, a thin-film, and the like. A Liquid Crystal Display (hereafter referred to a LCD) device has a viewing angle restriction, a long response time, and the like. However, the OLED display device is provided with features such as a wide viewing angle, a quick responding speed and the like. Accordingly, the OLED display device has been noticed as a next generation display.
Hereinafter, an operation principal of the display device using the OLED will be explained.
When power is supplied, electrons move and a current begins to flow. The electrons (−) from a cathode move toward an emitting layer by help of an electron-transfer layer, while holes (+: state of electrons released) from the anode move toward the emitting layer by help of a hole-transfer layer. The electrons and holes converged at the emitting layer of an organic material generate an exciton having higher energy state, and simultaneously create light when the exciton is fallen down to a lower energy state. A color of light varies according to what kind of the organic material the emitting layer is composed. A full color may be realized by each organic material emitting R, G, B colors. Contrary to the LCD with a simple function of open/closing pixels, the OLED utilizes self light-emitting organic materials.
Presently, the OLED display device as a thin-film type display device can apply a Passive Matrix (PM) driving method and hence an Active Matrix (AM) driving method, in the same method as the LCD in which has been used widely and commercially. The passive matrix driving method can have a simple structure and apply data exactly to each of the pixels. However, the passive matrix driving method is difficult to be applied to a large screen and a high-precision display. Accordingly, the development of the active matrix driving method has been actively proceeding.
A driving circuit of the OLED will be now explained with reference to FIGS. 1 and 2 according to a conventional active matrix driving method.
FIG. 1 is a schematic diagram illustrating the driving circuit of the OLED having a pixel circuit according to a conventional active matrix method.
Referring to FIG. 1, in the driving circuit of the OLED, a plurality of scan lines (X1, X2, X3, . . . , Xn) for selecting and unselecting the pixels 80 for a desired scan cycle (e.g., a frame period according to a NTSC standard) and a plurality of data lines (Y1, Y2, Y3, . . . , Yn) for supplying luminance information so as to drive the pixels 80 are arranged in a matrix type. The pixels 80 are arranged in each intersection portion in which the scan lines and the data lines are arranged in the matrix type. The respective pixels 80 are composed of a pixel circuit.
The scan lines (X1, X2, X3, . . . , Xn) are connected to a scan line driving circuit 20, and the data lines (Y1, Y2, Y3, . . . , Yn) are connected to a data line driving circuit 10. A desired image can be represented by selecting sequentially the scan lines (X1, X2, X3, . . . , Xn) by the data line driving circuit 10, supplying a voltage (or current) of the luminance information from the data lines (Y1, Y2, Y3, . . . , Yn) by the data line driving circuit 10, and filling repeatedly the voltage of the luminance information. In this case, the driving circuit of the passive matrix type OLED emits light only while light-emitting elements included in the respective pixels 80 are being selected, while the driving circuit of the active matrix type OLED continuously performs the light emission of the light-emitting elements even after the voltage (or current) supply of the luminance information is finished.
Thus, in the large screen and high-precision display, the active matrix type OLED is more superior to the passive matrix type OLED because the driving current level of the light-emitting element is low.
Hereinafter, a driving operation of the driving circuit of the OLED having the plurality of pixels 80 will be explained.
First, the scan line driving circuit 20 selects one of the scan lines (X1, X2, X3, . . . , Xn) and transmits a selecting signal. In the data line driving circuit 10, the data of the luminance information is transmitted to pixels arranged in transverse direction via the data lines (Y1, Y2, Y3, . . . , Yn). Then, the scan line driving circuit 20 transmits an unselected signal to the selected scan line, and then selects the next scan line (XN+1) so as to transmit the selected scan line signal. If the selection signal and the unselected signal are sequentially transmitted to the scan line, the driving circuit of the OLED can obtain a desired display by transmitting repeatedly the data.
FIG. 2 is a circuit diagram illustrating a typical pixel which is included in a driving circuit of an OLED according to an active matrix method.
Referring to FIG. 2, a pixel circuit for driving a pixel 80 includes two NMOS transistors T1 and T2, i.e., a first and a second active element, a capacitor CST, and an OLED. A gate terminal of the NMOS transistor T1 is connected to a scan line XN, a drain terminal thereof is connected to a data line YM, and a source terminal thereof is connected to the gate terminal of the NMOS transistor T2 and the capacitor CST. A source of the NMOS transistor T2 is connected to a positive pole (i.e., anode), and a drain terminal thereof is connected to a positive power source (VDD).
A cathode of the OLED is connected to a negative supply source (VSS). Thus, a current of the OLED is controlled by applying a voltage of the data line Ym to the gate terminal of the NMOS transistor T2 via the NMOS transistor T1.
Hereinafter, a driving operation of the pixel circuit will be explained.
When the gate terminal of the NMOS transistor T1 receives a selection signal from the scan line XN, the NMOS transistor T1 is turned on. At this time, a voltage corresponding to luminance information, which is applied to the data line YM by the data line driving circuit, is transmitted to the gate terminal of the NMOS transistor T2 via the NMOS transistor T1, and the luminance information voltage is stored in the capacitor CST. Even while the NMOS transistor T1 is turned off by receiving the unselected signal supplied from the scan line XNover one frame period, the voltage of the gate terminal of the NMOS transistor T2 is constantly maintained by the capacitor CST and thus the current flowing to the OLED via the NMOS transistor T2 is constantly maintained.
As such, in conventional pixel circuit, since the current flowing to the OLED is the same as the current flowing from the drain terminal of the NMOS transistor T2 to the source terminal, the current is controlled by the voltage of the gate terminal of the NMOS transistor T1, but is different from the magnitude of a desired current due to unevenness of an electrical characteristic or a characteristic deterioration of the NMOS transistor T2.
The thin film transistor used in the display device is an active element suitable for the large screen and high precision display. However, even though the thin film transistor is formed on the same substrate, there is a problem that a threshold voltage of the thin film transistor frequently has a voltage deviation of several hundreds of mV or more than 1 Volt.
For example, even though a same signal potential is supplied to the gate of the thin film transistor in different pixels, when the threshold voltage of the transistor included in each pixel is different, the current value flowing to the OLED deviates greatly from a value necessary for each pixel, and thus the high quality of the image necessary for the display device can not be obtained. The threshold voltage can not avoid some degree of variation according to a manufacturing company or products.
The respective products are necessary to determine how the data line potential is established for the driving current to be flown to the OLED according to parameter. However, it is difficult to be realistic in a manufacturing process of the display device.
Additionally, the driving current is greatly varied at an initial value of the threshold voltage due to the characteristic deterioration caused by a environment temperature and an usage for a long time. In this case, the definition or brightness of the display is greatly varied, thereby decreasing rapidly life of the display device.